This invention relates to a dual check backflow preventer designed to protect liquid supply lines from back-siphonage or back-pressure of non-potable liquids. Backflow preventers of this type have two independently acting and spring-loaded check valves constructed of a corrosion resistant material. Such devices also typically have an intermediate atmospheric vent to insure protection from backflow conditions. The backflow preventer of this application is described with reference to its use in a water supply circuit, which is a principal usage. However, it will be understood that the subject matter of this invention is not limited to use with water, but is applicable to any liquid circuit where backflow is not desirable or permitted.
During normal flow operation, the vent valve is closed, and the two check valves, which are in a flow series configuration, are open, allowing water to flow through the unit in a single direction. Each check valve is designed to hold a specified pressure in the direction of flow. When a backflow condition occurs, both check valves close and the atmospheric vent opens to permit air to enter the intermediate zone where the vent is located. In the event of back pressure, and if the second check valve is prevented from closing tightly, leakage is vented to atmosphere through the vent port.
Backflow preventers are subject to harsh use conditions, including wide temperature and water pressure variations, but are expected nevertheless to operate flawlessly over long periods of time, both during normal use conditions and when back-siphoning occurs. The relatively large number of components inherently provides potential leak paths, particularly around rubber seals, the diaphragm and swaged areas. In addition, a properly designed and operating backflow In accordance with another embodiment of the invention, preventer should permit full flow during normal operation with a minimum of flow turbulence.
Prior art backflow preventers include designs that are subject to premature wear and failure, leakage and flow-retarding turbulence. The present invention includes several novel features that correct these deficiencies.